Hitherto, an image intensifier (hereinafter, abbreviated to I.I.) has been used as a detector for capturing radiation images in a radiation imaging apparatus for medical diagnosis for use in fluoroscopic radiography. The radiation imaging apparatus using I.I. performs fluoroscopic radiography in such a manner that radiation image information based on radiation that has passed through an object is converted into optical information and then the optical information is intensified in luminance, condensed and picked up by a camera.
With advances in thin-film semiconductor technology of late, a flat panel detector described in European Patent Publication No. 0791964 has been practically used as a detector for capturing radiation images. The flat panel detector has a converting unit in which a plurality of pixels consisting of thin film semiconductors are arrayed on an insulating substrate made of glass, and the converting unit converts radiation image information into an electrical signal to provide image information. The pixel has a converting element which converts radiation into an electric charge and a switching element which transfers the converted electric charge. Known converting elements include two types: an indirect-converting type composed of a scintillator which converts radiation into light and a photoelectric transducer which converts the converted light into an electric charge; and a direct-converting type using a semiconductor material which directly converts radiation into an electric charge. Known switching elements include: a thin film transistor (hereinafter, abbreviated to TFT) composed of a thin film semiconductor; and an element using a thin film diode or the like. The use of a non-single crystalline semiconductor such as an amorphous semiconductor or a polycrystalline semiconductor in the pixel composed of the thin film semiconductor enables realizing a detector which is larger in radiographic area and lighter in weight than that using a conventional I.I. FIG. 7 shows an example of the equivalent circuit for the flat panel detector.
Such a flat panel detector has been used as a detector for capturing still images such as X-raying using film until now. At present, use of a flat panel detector as a detector for radiographing moving images such as fluoroscopic radiography and the like has come to be a matter of interest and study. Japanese Patent Application Laid-Open No. H11-009579 discloses a radiation imaging apparatus using a flat panel detector as a detector. The radiation imaging apparatus uses a flat panel detector which is lighter in weight than and superior in portability to the conventional detector using an I.I. as a detector, so that the flat panel detector is detachably mounted. In addition, a plurality of flat panel detectors different in visual field size (or radiographing area) are prepared, to permit use of a flat panel detector suited for a demanded visual field size, thereby providing a single apparatus which can be operated with plural visual field sizes.
FIGS. 8A and 8B show an example of a radiation imaging apparatus using such a flat panel detector. FIG. 8A is a schematic diagram of a stationary radiation imaging apparatus to be used with the apparatus fixed to the ceiling of a consulting room. FIG. 8B is a schematic diagram of a mobile radiation imaging apparatus. In FIGS. 8A and 8B, reference numeral 801 denotes a radiation generating unit which generates radiation such as X-rays; 802, a flat panel detector; and 803, a holding unit called a “C-type arm” for holding the radiation generating unit 801 and the flat panel detector 802. Reference numeral 804 signifies a display unit capable of displaying radiation image information derived by the flat panel detector 802; and 805, a bed for placing thereon an object. In addition, reference numeral 806 indicates a carriage which can carry the radiation generating unit 801, the flat panel detector 802, the holding unit 803 and/or the display unit 804 and has a structure capable of controlling them; and 807, a fitting unit for fitting the radiation generating unit 801, the flat panel detector, and the holding unit 803.